1. Technical Field
This invention relates generally to wireless local area networks (LANs), and, more particularly, to an apparatus and method for detecting when a computer user in a wireless LAN is about to move out of a service area of the LAN.
2. Related Information
Local area networks which allow a plurality of computers to communicate among themselves and with a network server are well known in the art. A detailed discussion of such networks, including communication standards and network topologies, is provided in the book Networking Personal Computers, 3rd edition, by Michael Durr and Mark Gibbs, copyright 1989 by Que Corporation of Carmel, Indiana, incorporated herein by reference.
As computers have become smaller and more portable (e.g., so-called "laptop" and "notebook" computers), computer users are able to move about more freely and, in particular, to operate their computers while traveling. Through the use of a modem and a cellular telephone, portable computer users have been able to communicate with other computers hundreds or thousands of miles away without the need to connect to a wire network. Cellular phone-based connections, however, presently have disadvantages such as high equipment cost, high telephone charges, and bandwidths limited to audio frequencies.
The use of wireless networks for communicating among computers is also well known in the art. Such networks allow computers to be moved within a particular physical area ("service area") without regard to physical connections among the computers. These networks may employ infrared light or radio wave propagation as the transmission medium. However, the use of these mediums may result in decreased transmission reliability for a variety of reasons such as radio frequency interference or transmission path obstructions. Therefore, such wireless networks often include one or more mechanisms to compensate for the decrease in transmission reliability. Such mechanisms may be preventive in nature, such as the use of spread-spectrum radio transmission to avoid radio frequency interference, or they may be corrective in nature, by detecting a transmission error and retransmitting data which was improperly transmitted.
The advent of wireless networks has benefited portable computer users. Although primarily developed to serve relatively fixed computer configurations in office buildings where the expense of installing coaxial cable or other more permanent network media would be expensive or destructive, wireless networks are increasingly being used to connect portable computers for purposes of user convenience. For example, a group of portable computers may communicate within an area served by the wireless network, allowing portable computer users in such a network to move freely around the area while maintaining communication with the network.
As a more specific example, in a large warehouse environment in which inventory is checked and maintained by computer, it may be desirable to provide one or more portable computers linked via a wireless network to a central computer, wherein one or more of the portable computers may be moved about the large warehouse and operated to update database entries based on inventory observations. As changes are made from a portable computer, they may be transmitted immediately via the wireless LAN to the central computer on the warehouse premises, instead of waiting until later and downloading such data. Conversely, data may be remotely retrieved from the central computer to the portable computers over a wireless network.
As another example, in a large retail store environment, it may be desirable to provide portable computers wirelessly linked to a central computer to monitor alarms, provide demonstrations, or maintain inventory. Office buildings provide yet another environment in which it may be advantageous to configure portable computers in a wireless network, so that portable computers may be taken into conference rooms, outside the building, or among offices.
In the foregoing environments in which portable computers are connected by a wireless network, a problem exists with regard to the degree of mobility. Unlike a cellular network with overlapping "cells" which allow a user to move between areas, conventional wireless networks only provide communication coverage over a limited area. This area is typically restricted by the distance between the server and the portable computers, or by obstructions such as large buildings or underground premises. Moving outside this area, or to an area in which communication is obstructed, results in disconnection from the network.
Unfortunately, there is usually no way for a portable computer user to detect when he or she is about to move out of range of the wireless network or is moving into an area in which signal obstructions may inhibit or prevent communication with the central server. Of course, if the portable computer user is presented with error messages indicating total loss of communication or an obvious lack of response from the central server, it is too late. Thus, a problem exists in that there is no accurate and reliable method of informing a user of a wireless portable computer linked to a network (hereinafter "mobile computer") that he is about to move out of the service area of the wireless network before the communications link becomes seriously deteriorated.
Methods of measuring signal strength as an indirect indicator of communication quality, while adequate for analog voice communications, do not accurately predict the quality of communications where data packets are transmitted relatively infrequently between computers. For example, erroneously transmitted data packets can be easily retransmitted between computers with little or no negative effect to the mobile computer user, and thus the communications quality (as perceived by the computer user) can be high even when signal strength is low or fading. Thus, using signal strength (or a related measurement) to determine communications quality in a wireless network is disadvantageous.
Conventional methods of measuring communication quality also do not allow a computer user to adjust the means by which the communications quality is determined in order to compensate for different data communications environments such as noisy environments or different data communications speeds. Allowing a user to individually adjust how the communications quality is determined would allow the user to more accurately monitor the actual communications quality (i.e., the ability to exchange computer data) under various conditions. As an example, it would be desirable to allow an individual user to adjust one or more thresholds which are used to determine that the communications quality is deteriorating, so that severe deterioration could be tolerated where data exchanges are infrequent.
The inventors of the present invention have found that determining the communications quality between a mobile computer and a central computer server is best effected by measuring the packet transmission error rate rather than the quality of the signal on which the data is modulated. The related art contains other limitations relating to wirelessly networked computers which will become apparent through the following discussion.
Conventional technology provides partial solutions to measuring communications quality between two or more devices which communicate wirelessly. For example, cellular radio telephone devices may include an alarm to indicate to a user that the strength of a radio frequency signal has decreased below a certain value when the telephone user reaches the boundary of a zone. However, as pointed out previously, using signal strength to evaluate the communications quality imposes disadvantages where digital data packets are transmitted between computers. Moreover, the user has no control over how the communications quality is measured, thereby producing undesirable results in noisy environments where, for example, a high error rate may be tolerable.
The related art also generally discloses the use of bit error rates measured between a base station and a mobile receiver for selecting, when a predetermined threshold is reached, a neighboring zone transmitting at a lower error rate. However, such devices do not provide an indication to the user that he is about to move out of a service area, thereby allowing him to move back within the area before communications with a base station is lost. Moreover, such devices do not allow the user to adjust the means by which communications quality is determined, thereby incurring the disadvantages outlined above.
The related art also allows wireless communications among a group of computers and peripherals using packet switching protocols which enable two-way error free transfer of data, while accommodating breaks in the communications path. Data packets may be retransmitted when errors are detected, the errors being detected through the use of a cyclical redundancy check (CRC). When a predetermined number of retransmission attempts has been made, a particular link may be deemed to be "down". However, such systems do not measure the transmission error rate (i.e., errors as a function of time), thereby limiting their ability to detect changes in transmission quality over time. Moreover, because error thresholds are predetermined, the systems cannot adapt to noisy environments or circumstances in which the transmission quality varies over time. No warning or alert is provided to a user in such a network that the user is approaching the outer limits of a service area, which might otherwise allow the user to move back within the service area to prevent further deterioration or disconnection from the network. Thus, what is needed is a system and method for solving these problems.